Mercury control using moderate-temperature dissociation of halogen compounds

ABSTRACT

A system and method is provided for the removal of mercury from flue gas. Effective removal of mercury is obtained by oxidation of elemental mercury, with highly reactive halogen species derived from dissociation of halogen compounds at moderate temperatures brought into contact with the flue gas with or without the addition of carbon.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119 to co-pending,commonly owned U.S. provisional patent application No. 60/947,543,attorney docket number EER.P0036P, filed on Jul. 2, 2007, entitled“MERCURY CONTROL USING MODERATE-TEMPERATURE DISSOCIATION OF HALOGENCOMPOUNDS,” (pending) which is incorporated by reference herein.

This invention was made with U.S. government support under Contract No.CR-83092901-0 awarded by the Environmental Protection Agency. Thegovernment has certain rights in this invention.

FIELD OF THE INVENTION

The present invention relates to mercury emission control systems. Inparticular, the invention is drawn to a system and method for removingmercury from a flue gas stream using dissociation of halogen compounds.

BACKGROUND OF THE INVENTION

Because mercury (Hg) is toxic and can bioaccumulate in the human body,mercury emissions have become a health concern. The U.S. EnvironmentalProtection Agency (EPA) has recently determined that regulation of Hgemissions from coal-fired electric power plants is necessary andappropriate. The various state and federal mercury regulations havecreated an urgent need to develop effective control technologies.Mercury in flue gas can be captured by injection of powdered sorbentswhich are removed by subsequent particulate collection devices. Althoughsorbent injection is, so far, the most mature control technology, theamount of sorbent needed to serve the U.S. market is expected to be solarge that there is still a need to develop new methods to minimizechanges required for utilities and to reduce costs associated withcapital equipment and carbon injection.

SUMMARY OF THE INVENTION

A method of the invention is provided for removing mercury from a fluegas stream of a coal combustion system, including generating adissociated halogen gas from a reactive precursor at a temperature inthe range of 60°-400° C., introducing the dissociated (atomic) halogengas to the flue gas stream to allow the dissociated halogen gas to reactwith mercury present in the flue gas stream at a temperature in therange of 60°-400° C., and capturing the mercury using a pollutantcontrol device.

Another embodiment of the invention provides a mercury control systemfor a coal combustion system having a coal combustor and a pollutantcontrol device, the mercury control system including a halogendissociation unit for generating a dissociated halogen gas, the halogendissociation unit being in communication with a flue gas stream of thecoal combustion system, and wherein the mercury control system isconfigured to expose the dissociated halogen gas to mercury present inthe flue gas stream, allowing the dissociated halogen gas to react withthe mercury at a temperature in the range of 60°-400° C. and be capturedby the pollutant control device.

Other features and advantages of the present invention will be apparentfrom the accompanying drawings and from the detailed description thatfollows below.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and notlimitation in the figures of the accompanying drawings, in which likereferences indicate similar elements and in which:

FIG. 1 is a schematic diagram of a halogen dissociation system appliedin a coal-fired system.

FIG. 2 is schematic diagram of another halogen dissociation systemapplied in a coal-fired system.

FIG. 3 is a flowchart illustrating one example of the operation of thepresent invention.

DETAILED DESCRIPTION

To understand the present invention, it is helpful to understand variouschemical reactions that take place in a coal-fired combustion system.Flue gas constituents, especially halogens or halides, have greatimpacts on mercury fate in flue gas. The chlorine in coals or halogencompounds that have been added to the fuel are converted in the furnaceto the atomic form, but being highly reactive, react with flue gascomponents and each other to form the molecular forms. Reactions withwater vapor, SO₂, and other components occur and form products such asHCl, SO₂Cl₂, and Cl₂. As the flue gas cools, reactions of atomic ormolecular halogens with elemental mercury may occur. Heterogeneousreactions with or on particulate may also occur in addition to gas-phasereactions.

Reactions of atomic chlorine generated in a furnace are kineticallylimited and heavily depend on temperature-time profile. The issue is howmuch and what form is effective for oxidation of Hg⁰ in the gas phase.Rate constants for Hg+Cl atoms can be determined by measuring theformation of HgCl using a spectroscopic (279-nm) method. Thesecond-order rate constant for this Hg(I) species is about 1-3×10⁻¹¹ cm³molecules⁻¹ sec⁻¹. This reaction could be followed by a second reactionto form HgCl₂. Using indirect methods, second-order rate constants forHg⁰ with halogen species can be determined as follows: with Cl atoms,1.0×10⁻¹¹ cm³ molecules⁻¹ sec⁻¹; with Br atom, 3.2×10⁻¹² cm³ molecules⁻¹sec⁻¹; with Br₂, 9×10⁻¹⁷ cm³ molecules⁻¹ sec⁻¹; with Cl₂, 2.6×10⁻¹ cm³molecules⁻¹ sec⁻¹. Thus the atomic Cl rate constant is about 4 milliontimes higher than Cl₂. However, under a typical temperature profile of acoal-fired utility plant, the atomic Cl generated in the combustion zonehas already reacted with other flue gas constituents or itself before itcould oxidize elemental mercury at the required lower temperatures. Itis well known that the thermodynamics are favorable for formation ofoxidized mercury compounds at temperatures less than about 400° C.,owing to the instability of oxidized the mercury compounds.

Because halogen reactivity with mercury can be an important factor inmercury control, basic research in this area has been conducted. Forexample, the oxidation of Hg with injected HCl and Cl₂ has been studiedusing a quenching system comprising a gradient temperature reaction tubefrom which samples could be withdrawn for analysis. No oxidationoccurred using realistic quench rates with 100 ppm HCl. This isexpected: HCl is not an oxidizing agent, since it is already in a highlyreduced form. Using a composition containing 50 ppm Cl₂ gave only 10%oxidation of Hg⁰, while very large amounts (500 ppm) of Cl₂ gave 92%oxidation. The implication is that the more reactive atomic chlorine wasnot available in the system. In another example, HCl and Hg(II) acetatewas injected into a natural gas flame and obtained oxidation dataconsistent with the formation of atomic chlorine and subsequent reactionof a superequilibrium concentration of atomic chlorine with Hg⁰ attemperatures between 400° and 700° C. Using a very fast (10×) quenchrate, up to 40% oxidation at 300 ppm Cl can be achieved.

Research recently demonstrated enormous mercury enrichment in ash whenHCl was fed into a high-temperature environment followed by a super-fastquenching rate of 5400° C./s. Only 6% of the elemental mercury was notoxidized and converted to particulate forms. This experiment impliesthat atomic chlorine generated in the hot zone was still available atlower temperatures, owing to the fast quench rate and, thus, oxidizedthe mercury at a lower temperature where Hg—Cl reactions are mosteffective. While most of the mercury was on the ash, it was not clearwhether oxidation occurred in the gas phase or solid phase, or where onthe solid phase. The consensus is that much of the mercury is oxidizedby reactive halogens in a heterogeneous reaction on a carbon particulatesurface. The initial product of the atomic chlorine reaction with Hg isHgCl, which would readily collect on ash, carbon, or sorbent particlesor react with other species or itself.

Further pilot-scale experimental data showed that the atomic halogenradicals formed at high temperatures can not only significantly enhancemercury oxidation, but also improve the reactivity of mercury withactivated carbon. The atomic and/or molecular halogen species areformed, at least momentarily, in the high-temperature environment athigh rates with mercury both as gas-phase and solid-gas interactions.Flue gas-quenching rate plays a critical role in the mercury-halogenchemistry. An excessive flue gas-quenching rate will preserve thereactive halogen species formed in the high-temperature zone for ongoingmercury oxidation and gas-to-particle conversion. However, rapidquenching is difficult to achieve in a practical application likeutility flue gas treatment.

Recent bench-scale and pilot-scale experimental data demonstrate thathalogenation of carbon resulted in vastly improved mercury capturekinetics and overall control performance. Halogen elements are able toactivate the carbon surface to increase the reactivity of the carbon orthe elements form more reactive forms under certain conditions. It ispossible to generate the desired halogen radicals via high energydissociation from relatively stable salts and compounds.

Generally, the invention is a system and method for enhancing sorbent(and ash) reactivity and/or for oxidation and removal of gaseous mercuryin a flue gas with or without a sorbent. It obviates the fast quenchingneeded for effective homogeneous mercury oxidation by providing atomic(dissociated) halogen at a low enough temperature to effect the mercuryoxidation to stable products that can be readily captured in a scrubberor ash particulate collector device. In addition, the generated reactiveatomic halogens described herein can impregnate commercially availablecarbons and other sorbents in their feed line into coal-fired plants.The dissociated halogen-treated sorbents have an enhanced reactivitywith mercury and improve mercury capture in flue gas.

In one example, the present invention utilizes a moderate temperatureprocess to dissociate halogen-containing materials into atomic halogenforms which are very strong mercury oxidants. The highly reactivehalogen used in the reactive portion of the mercury removal process canbe used in any desired way. In one example, the dissociated halogen gasis passed directly into a flue gas stream to oxidize elemental mercuryand/or transform gaseous mercury into a particulate-associated form. Inanother example, the dissociated halogen gas is generated by directinjection of the halogen compound into the flue gas at or above thedecomposition temperature. In another example, the dissociated halogengas is passed into a chamber or duct containing a carbon form thatprovides a reactive surface for the mercury oxidations.

In one example, a moderate-temperature halogen dissociation unit usedwith the present invention can be operated at temperatures ranging fromapproximately 60° to 400° C. to ensure an efficient dissociation. Theactual temperature used is dependent upon the halogen materials used inany particular embodiment of the present invention. In one example, thereactive process of the moderate-temperature halogenated gas describedherein can be applied anywhere in a coal-fired system from the boileroutlet to the inlet of a particle or flue gas desulfurization controldevice. Transport of the reactive dissociated halogen forms to thereaction process zone where it encounters the main flue gas streamoccurs rapidly, in one example. The loss of temperature/energy of thedissociated gas should also occur rapidly either during or prior toentering the reaction zone. This maintains a superequilibrium of theatomic forms and minimizes unwanted side reactions. The generatedreactive halogens described herein may impregnate carbons that are insitu-generated either in the halogen dissociation unit or in a separatecarbon generation unit. The halogen-treated carbons have an enhancedreactivity with mercury and improve mercury capture in flue gas. In oneexample, the moderate-temperature halogen dissociation may be achievedby any heating and/or photolytic process to include convective,conductive, and radiative (light, microwave, Rf, arc, etc.) energysources. In one example, the halogen-containing materials may be organicor inorganic chemical compounds, and they can be solid, liquid, and gasphases.

The present invention applies concepts relating to the formation andrapid transfer of radicals to a combustion flue gas into a practical andeffective system and method for mercury control in a utility (orindustrial) flue gas stream. Because of its high reactivity, it isdifficult to preserve atomic chlorine generated in the furnace so thatit will be available for reaction with Hg⁰ at lower temperatures wherethe resulting mercury chloride will be stable. The present inventiongenerates the atomic chlorine and other halogens and halogenated radicalspecies at moderate temperatures either in the gas stream or in aseparate chamber and introduces them to the flue gas stream at lowtemperatures either in the gas stream or in a separate chamber to areactive carbon surface so that the long reaction time with other fluegas components is avoided.

Once converted to oxidized mercury by the reaction with the reactivehalogen atoms, the Hg(II) halide species are captured readily viareaction on the solid ash phase or with an injected sorbent material,such as activated carbon, lime, calcium silicate, or other basicparticulate. Alternatively, the gas stream containing oxidized mercurycan be cleaned with a wet scrubber, where the mercury is converted to astable solid or liquid form.

One concept employed in the present invention is that certain organicand inorganic halides have a relatively moderate to low decompositiontemperature (<500° C.). The thermal and photochemical decompositions oforganic halides result in breakage of the carbon-halogen bond andproduce various halide products, depending on the nature of the halogenas well as the organic portion of the molecule. Some of thedecompositions produce hydrogen halides (HX) via a unimolecularmechanism that simultaneously splits out the HX without formation ofatomic halogen (X.) or molecular halogen (X₂). Decompositions of othercompounds do involve the generation of atomic halogen and produce HX andother species in subsequent reactions of the radicals. It is especiallythe latter type of decompositions that could be used to furnish reactivehalogen radicals for enhancement of sorbent reactivity or oxidation ofmercury. Thus, heating these organic halides to temperatures above thedecomposition temperature (typically, for example, 260°-500° C.) in agas stream would generate in situ atomic halogen species that could beused to enhance sorbent reactivity or oxidation in a flue gas stream.

Organic bromides and chlorides, for example di- and polyhalides, willdecompose to atomic halogen and other radicals at moderate temperatures(e.g., <500° C.). Iodides decompose readily with formation of I atomintermediates, since the C—I bond energy is so low. Fluorides typicallydecompose with rupture of the carbon-carbon bonds since the C—F bondenergy is very high, and no atomic fluorine or HF is formed, althoughreactive fluorocarbenes are produced. Owing to the lower bond energy ofcarbon-bromine bonds, many alkyl bromides decompose via radical typedecompositions. Examples of some reactive organobromine compounds aregiven in Table 1. Table 1 shows radical decomposition of variousorganobromine reactants, and an exemplary temperature range fordissociation.

TABLE 1 Lowest Temp. (° C.) Reactant of Range for DissociationBromomethane   500 Bromoethane   310-476 1-Bromopropane   300-380 AllylBromide   320-380 1-Bromo-2-Methylpropane   300-3901-Bromo-2-Chloroethane   307-358 1,2-Dibromoethane   300 t-ButylHypobromite <300, Light

Many alkyl chlorides will produce radicals when heated at moderatetemperatures. Examples of the radical chlorine decompositions are listedin Table 2. Table 2 shows examples of various exemplary reactants theexpected temperature of decomposition.

TABLE 2 Reactant Expected Temp. (° C.) of Decomposition Chloroform   450Carbon Tetrachloride   554 1,2-Dichloroethane   362 1,2-Dichloroethene  370 Trichloroethane   380 Tetrachloroethane   262 Allyl Chloride   370Oxalyl Chloride   260 t-Butyl Peroxychloroformate    60 MethylChlorosulfite   380 Alkyl Hypochlorites <300, Light Phosgene <450

Homolytic dissociation of the carbon-halogen bond to form atomic halogenand organic radicals requires substantial activation energy. Thus, mosthaloorganic compounds dissociate via a nonradical unimolecular mechanismto form the hydrohalogen acid (HX) where the HX splits off without anyradical formation. The HX does nothing for oxidizing mercury. But theactivation energy for splitting to radicals is lower when the organicradical that forms is stabilized, as in the allyl halide dissociationand in di- or polyhalo compounds dissociations where the double bond orthe remaining halogen, respectively, stabilize the adjacent carbonradical. Thus, the decomposition of allyl halides proceeds by a radicalnon-chain mechanism (Eq. 1, below), where many halogen atoms are formedat a lower reaction temperature. The propenyl radicals that arecogenerated in the initial reaction mostly react with themselves on thewalls of the container to form other organic compounds and carbondeposits. The dihaloorganic compounds dissociate via a third mechanism,the radical chain mechanism. Like the allyl halide decompositions, someatomic halogen is generated, but the halogen atoms attack theundissociated molecules by abstracting hydrogens and forming HX and anew stabilized radical (Eq. 2, below). These reactions have very lowactivation energies, so the predominant reactions are a set of chainreactions to produce HX and new haloorganics. Mercury atoms would haveto compete with the haloorganics for the small concentration of halogenatoms. This would not work as well as the allyl unimoleculardissociations, because the low activation energies for the haloorganicreactions will consume atomic halogen and form HX.

CH₂═CH—CH₂X→CH₂═CH—CH₂.+X  [Eq. 1]

X.+CH₂X—CH₂X→CH₂X—CHX.+HX  [Eq. 2]

Allyl halides decompose via dissociation to halogen atoms without havingto rely on the chain mechanism. An important point is that dilution withflue gas will inhibit the radical chain reaction mechanism, but will notinhibit the unimolecular reaction of allyl halides. Therefore, allylhalides are desired reagents for the introduction of large amounts ofatomic halogen to the duct at an appropriate low-moderate temperature sothey can oxidize mercury to stable species. In one example, allylhalides can be injected into a duct prior to an electrostaticprecipitator device at a temperature in the range of approximately320°-420° C. In one example, sulfuryl chloride is injected into the ductprior to an electrostatic precipitator and/or fabric filter and/orscrubber at a temperature in the range of approximately 100°-400° C.Other examples are also possible.

The use of activated carbon for mercury removal may not be desired forsome utilities where contamination of the ash with carbon is notdesired. The described organohalogen dissociation method for oxidationand capture on ash particulates or scrubbers can work well for many ofthese utilities. In other cases, higher removal rates may be achieved byusing a combination of the atomic halogen produced bymoderate-temperature dissociation of organic halides in conjunction witha sorbent. Atomic halogens appear to interact with elemental mercury ona carbon sorbent surface so as to promote the oxidation and reactivityon sorbent surfaces resulting in improved subsequent capture. It isknown that a sorbent (carbon) surface can inhibit the radicaldecomposition of alkyl halides, so there seems to be some stabilizingfactor operating on the radicals. Thus if the radicals are generated offof the surface, they may still be stabilized on contact with the surfaceand be available for reaction with the elemental mercury that alsocontacts the surface. Glass does not exhibit this effect, so it isunlikely that ash particles will stabilize the radicals as effectivelyas carbon.

The radical dissociation of an alkyl halide may be initiated by radicalspecies such as NO. Thus dissociation in a NO stream or in the presenceof flue gas containing NO may promote the radical dissociation mechanismfor certain alkyl halides, but this does not necessarily generate morehalogen atoms. In some cases, NO inhibits the chain reactions byscavenging radicals.

Photolytic dissociation of organic halides proceeds by a radicaldissociation. For alkyl chlorides and bromides, considerable energy isrequired, and very short wavelengths would be necessary. However,halogenated carbonyl compounds, such as chloroacetone, absorb light oflower energy (for example, 313 nm), and thus gas-phase photodissociationof the halocarbonyl compounds may be a more practical method for halogenatom generation than alkyl halide photodissociation. Heating thehaloketone typically increases quantum yields of photoproducts and maybe applied to the flue gas reaction. Tertiary-butyl hypochloritedecomposes in sunlight via a radical reaction.

Certain inorganic nonmetal halides decompose to form reactive halogenradicals. These include halides of phosphorus, selenium, sulfur,silicon, and nitrogen. Examples of the compounds include Se₂Br₂, SeOBr₂,PBr₃, PBr₅, POBr₃, and SiBr₄, with or without promoters in the gasstream such as NO.. A very inexpensive and convenient precursor issulfuryl chloride (SO₂Cl₂), which is a low-boiling liquid thatdissociates at about 100° C. Some of these inorganic halides are solids,but are soluble in organic solvents and, therefore, can be injected as asolution into the gas steam or into a carbon dispersion at anappropriate temperature to dissociate the halogens. Dissociationtemperatures vary widely, but some are below 50° C. Dissociationproducts include atomic halogen radicals, bimolecular halogens, andvarious other reactive radicals containing halogen attached to theheteroatom.

The thermal and photochemical dissociations of organic fluorides resultin breakage of the carbon-carbon bonds and produce various fluorocarbonproducts. Owing to the high strength of the carbon-fluorine bond,fluorine atoms or HF are not typically liberated during pyrolysis offluorocarbons. Instead, the thermal decomposition results in cleavage ofcarbon-carbon bonds and formation of fluorocarbon radicals, oftenincluding difluorocarbene (CF₂:). The latter type of dissociations couldbe used to furnish reactive electrophilic radicals for oxidation ofmercury. Pyrolysis of organofluorine compounds containing chlorine,oxygen, and other heteroatoms behaves similarly. Thus heating theseorganic halides to temperatures above the dissociation temperature(typically 260°-500° C.) in a gas stream, preferably an inert gas,generates difluorocarbene and other radicals such as CF₃ in the streamthat oxidize elemental mercury in a flue gas stream when contacted withthe organofluoride decomposition stream. In some cases where there is noC—C bond to break, such as formyl fluoride (125°-200° C.), atomicfluorine is liberated. Examples of the fluorocarbon decompositions arelisted in Table 3. Table 3 shows examples of radical decomposition ofvarious organofluorine reactants and the expected temperature ofdissociation.

TABLE 3 Radical Decomposition of Organofluorine Reactants ReactantExpected Temp. (° C.) of Dissociation Chlorodifluoromethane 425Hexafluoroacetone 570 Teflon 360 Trifluoroacetic Acid 300Trifluoroacetyl Fluoride 570

Difluorocarbene is highly electrophilic, so it reacts with elementalmercury, a Lewis base, resulting in its oxidation and conversion to anorgano Hg(I) radical, i.e., HgCF₂.. This is reactive to any of a varietyof species in the flue gas, such as HCl, SO₂, and H₂O and forms organoHg(II) species, such as ClHgCF₂H, etc.

The utilization of fluorocarbenes produced by moderate-temperaturedissociation of organic fluorides can be used for mercury capture inconjunction with a sorbent. The interaction of the carbene withelemental mercury on a carbon sorbent surface promotes the oxidation andsubsequent capture of mercury. A carbon surface can inhibit the radicaldecomposition of alkyl halides, so there seems to be some stabilizingfactor operating on the radicals. Thus, if the radicals are generatedoff of the surface, they are stabilized on contact with the surface andavailable for reaction with the elemental mercury that also contacts thesurface.

FIG. 1 is a schematic block diagram of one example of a halogendissociation system applied to a coal-fired system 10. The coal-firedsystem 10 illustrates a coal combustor 12, an air preheater 14, apollutant control device 16, a stack 18, and various ductsinterconnecting the system 10. Arrows represent the direction of gasflow through various places in the system 10. The coal combustor 12 maybe comprised of any desired type of device, such as a pulverized coalburner, a stoker burner, a fluidized-bed burner, or any other coalcombustor or gasifier used in a coal-fueled system. Generally, coal isinjected introduced to an input of the coal combustion system, where itis burned to generate heat. The flue gas is heated by the air preheater14 and routed to the pollutant control device 16, where variouspollutants are removed before releasing the flue gas to the atmospherevia the stack 18. For clarity, various other components present in atypical coal-fired system are not shown or described.

FIG. 1 also shows a halogen dissociation unit 20. The halogendissociation unit 20 can be installed at any location between the boileroutlet and the inlet of the pollutant control device 16. In the exampleshown in FIG. 1, the halogen dissociation unit 20 is installed betweenthe air preheater 14 and the pollutant control device 16. The halogendissociation unit 20 may operate at temperatures that are, in oneexample, less than 500° C. The halogen dissociation unit 20 may beheated or energized in any desired manner (not shown), includingelectric heating. Halogen-containing materials may enter thedissociation unit 20 in any of their physical forms (i.e., gas, liquid,or solid) and decompose into atomic, molecular, and/or radical forms.The decomposed halogen species will then convey directly into the mainflue gas stream 22. The carrier gas for the halogen compounds mayinclude mixtures containing NO or NO₂, or any other suitable gas. Thereactive halogens or halo radicals will efficiently oxidize elementalmercury vapor present in the main flue gas and/or enhance sorbentreactivity for improved subsequent mercury capture and/or convertgaseous mercury into particulate-associated forms, which can becollected by the pollutant control device 16.

FIG. 2 is a schematic block diagram of another example of a halogendissociation system applied to a coal-fired system 10. The coal-firedsystem 10, like the system shown in FIG. 1, illustrates a coal combustor12, an air preheater 14, a pollutant control device 16, a stack 18, andvarious ducts interconnecting the system 10. In the example illustratedin FIG. 2, the halogen compound and/or sorbent (e.g., an Alkalinesorbent) is injected directly into the flue gas at an appropriatetemperature (for example, 150°-500° C.) depending on the dissociationtemperature, so that thermal decomposition occurs instantly and radicalsare immediately available for reaction with mercury present in the fluegas. The oxidized mercury products are removed in a pollution controldevice following the point of injection at a temperature lower than theinjection temperature. This example also illustrates that this systemallows one to choose an appropriate halogen compound for an optimalinjection location for mercury removal at a given plant configuration.

In another example, the invention includes the addition of a photolysislamp 30 to the duct following the point of halide injection. Theoptional photolysis lamp 30 is illustrated in FIGS. 1 and 2 by a dashedbox. Photodissociation of the alkyl halide to atomic halogen istherefore accomplished prior to mixing with the ash-containing gasstream.

Another alternative configuration of the current invention is alsodepicted in FIGS. 1 and 2. In this example, carbon black, char, or sootare in situ generated in a separate unit 32 by either thermal or arcprocesses, for example. In these examples, the in situ-generated carbonblack, char, or soot is injected and is treated by the reactive halogensduring their transit to the main flue gas duct 22. This embodiment canalso be applied to dry sorbent (activated carbon) injectiontechnologies. Injected particulates may also include alkaline compounds,such as CaO, CaSiO3, etc.

FIG. 3 is a flow chart illustrating an exemplary process for using oneexample of the present invention. This example will be described in thecontext of a mercury control system for a coal combustion system. Theprocess begins at step 3-10 where a dissociated halogen gas is generatedfrom a halogen compound. As described above, the halogen gas can begenerated in any desired manner. In one example, the halogen compound isdissociated using a moderate temperature (e.g., a temperature less than500° C.) process that dissociates halogen-containing material intoatomic halogen forms. At step 3-12, the dissociated halogen gas isexposed to mercury present in the flue gas stream of the combustionsystem. As described above, the halogen gas may oxidize elementalmercury and/or transform gaseous mercury into a particulate-associatedform. If desired, the dissociated halogen gas can be mixed with a carbonform that provides a reactive surface for the mercury oxidation.Finally, at step 3-14, the mercury (now oxidized and/orparticulate-associated) is captured in a pollutant control device, suchas an ESP.

While preferred embodiments of the invention have been shown anddescribed, modifications thereof can be made by one skilled in the artwithout departing from the spirit and teachings of the disclosure. Theembodiments described herein are exemplary only, and are not intended tobe limiting. Many variations and modifications of the inventiondisclosed herein are possible and are within the scope of the invention.

Following are two specific examples of applications of the presentinvention to help in understanding the operation and advantages of theinvention. Many other examples are also possible.

In a first example, the invention was tested for mercury oxidation andremoval in a pilot-scale boiler by introducing alkyl halides (halogenprecursor) into flue gas containing elemental mercury and determiningthe removal of mercury from the gas stream using continuous mercuryemission monitors. In this example, an organic halide mixture composedof allyl iodide and tetrachloroethane (wt ratio 1:1) was injected into aflue gas from combustion of a Powder River Basin subbituminous coal. Theflue gas contained approximately 10 μg/m³ of elemental Hg. The alkylhalide mixture was pumped at 1 mL/min into a duct with the flue gasflowing at 132 scfm at 800° F. (427° C.). No carbon or basic sorbent wasinjected. The mercury concentration was determined downstream afterparticulate collection. Less than 3 μg/m³ of mercury remained in theflue gas. Previous experiments at this temperature at the same flow rateshowed no removal with no injection of organohalide reagent and noremoval with injection of the tetrachloroethane at 1 mL/min. Thus theallyl iodide was responsible for oxidation and removal of 70% of themercury from the gas stream.

In a second example, the demonstration test was conducted at aparticulate test boiler equipped with an electrostatic precipitator(ESP). A series of experiments introduced hexane solutions of allyliodide (RI), allyl chloride (RCl) and allyl bromide (RBr) vaporssequentially through a preheater into the duct with and withoutco-injection of activated carbon (AC) (in this example, Norit HG), whichwas expected to remove mercury from the gas phase. The pumping rate forthe allyl halide solutions was adjusted to provide a constantconcentration of 25 ppm of halogen in the flue gas, assuming completeconversion of the allyl halide to atomic halogen. The actual halogenconcentration in the flue gas was not measured, however, so the 25 ppmrepresents a theoretical concentration. Total gaseous mercury andelemental mercury were determined at the inlet and at the outlet of theESP by continuous mercury monitoring. The mercury measurements weretaken prior to and after allyl halide injection and also after addingboth allyl halide and activated carbon. The results are summarized inTable 4, below.

Prior to the addition of the allyl iodide, there was no capture acrossthe ESP, and no additional oxidized Hg (both 10% Hg(2+)), thus no netoxidation across the ESP (see line 2 of Table 4). The addition of allyliodide resulted in minimal capture in the ESP, owing to the lowattachment rate to particulates, but much more of the Hg in the gasphase was oxidized (41% of the total) (see line 3 of Table 4). Theaddition of both allyl iodide and AC, however gave 78% capture acrossthe ESP (see line 4 of Table 4).

Prior to the addition of allyl chloride, again there was no captureacross the ESP (see line 5 of Table 4). After the addition of allylchloride, the capture increased to 17%, and the oxidized Hg in the gasphase increased to 19% of the total (see line 6 of Table 4). Theaddition of allyl chloride and activated carbon gave 60% capture (seeline 7 of Table 4).

Prior to the allyl bromide addition, the capture across the ESP washigher (24%) compared to 0% in prior experiments (see line 8 of Table4). This may have been a residual effect of halogen or carbon in theduct. After the addition of allyl bromide, the capture increased to 44%,although the oxidized Hg in the gas phase was low (see line 9 of Table4). After the addition of allyl bromide and AC, the capture increased to79%, and the oxidized mercury in the gas phase to 25% (see line 10 ofTable 4).

Clearly, the capture efficiencies are remarkably good for an ESPconfiguration, especially for allyl iodide and allyl bromide. This isconsistent with the increase of oxidized mercury in the gas phase, whichdecreases in the order RI>RBr>RCl. This order is expected from the knownreactivity of the halogen atoms toward flue gas constituents, so thatchlorine atoms are depleted more rapidly by the flue gases compared toBr and I. This depletion may also occur in the self-reactions (Cl2formation and HCl formation) occurring during the transit from theheater to the duct. To make the allyl chloride more effective, thisdistance (from the heater to the duct) may be minimized.

TABLE 4 Hg Hg Total(g) Capture Capture Elem(g) Hg(2+)(g) Experiment(μg/m³) (μg/m³) (%) (μg/m³) (%) ESP Inlet 6.3 5.7 10 ESP Outlet withoutRI 6.3 0 0 5.7 10 addition ESP Outlet with RI 6.1 0.2 3 3.6 41 additionESP Outlet with RI & 1.4 4.9 78 ND — AC addition ESP Outlet without 6.30 0 ND — RCl addition ESP Outlet with RCl 5.2 1.1 17 4.2 19 addition ESPOutlet with RCl 2.5 3.8 60 ND — & AC addition ESP Outlet without 4.8 1.524 4.7  2 RBr addition ESP Outlet with RBr 3.5 2.8 44 2.6 25 additionESP Outlet with 1.3 5.0 79 ND — RBr & AC addition ND = Not Determined

In the preceding detailed description, the invention is described withreference to specific exemplary embodiments thereof. The majordifference between the detailed examples is that the liquid precursor isinjected directly into the flue gas stream in the first example where itevaporates quickly and dissociates, whereas in the second example, theprecursor was dissociated in a gas stream prior to its introduction intothe flue gas. Various modifications and changes may be made theretowithout departing from the broader spirit and scope of the invention asset forth in the claims. The specification and drawings are,accordingly, to be regarded in an illustrative rather than a restrictivesense.

1. A method of removing mercury from a flue gas stream of a coalcombustion system, comprising: generating a dissociated halogen gas froma reactive precursor at a temperature in the range of 60°-400° C.;introducing the dissociated (atomic) halogen gas to the flue gas streamto allow the dissociated halogen gas to react with mercury present inthe flue gas stream at a temperature in the range of 60°-400° C.; andcapturing the mercury using a pollutant control device.
 2. The method ofclaim 1, wherein the dissociated halogen gas oxidizes the mercurypresent in the flue gas stream.
 3. The method of claim 1, wherein thedissociated halogen gas transforms the mercury present in the flue gasstream into a particulate-associated form.
 4. The method of claim 1,wherein the mercury is captured using a particulate control device suchas an electrostatic precipitator and/or fabric filter and/or scrubber.5. The method of claim 1, wherein the halogen precursor is allyl iodide.6. The method of claim 1, wherein the halogen precursor is allylchloride.
 7. The method of claim 1, wherein the halogen precursor isallyl bromide.
 8. The method of claim 1, wherein the halogen precursorare organic di- and polyhalides including fluorocarbons, and inorganicnon-metal halides with low decomposition temperatures.
 9. The method ofclaim 1, further comprising injecting particulates into the flue gasstream.
 10. The method of claim 9, wherein the particulates arecomprised of activated carbon or alkaline compounds (CaO, CaSiO3). 11.The method of claim 1, further comprising exposing the precursor halogengas to light from a photolysis lamp.
 12. The method of claim 1, whereinthe dissociated halogen gas is generated at a temperature in the rangeof 60°-400° C.
 13. The method of claim 1, wherein the dissociatedhalogen gas is generated from an organic chemical compound.
 14. Themethod of claim 1, wherein the dissociated halogen gas is generated froman inorganic chemical compound at a temperature in the range of 60°-400°C.
 15. The method of claim 1, wherein the point of injection of thereactive precursor into the duct is at the appropriate temperaturecorresponding to the dissociation temperature of the precursor and belowthe decomposition temperature of oxidized mercury.
 16. The method ofclaim 15, wherein the allyl halides are injected into the duct prior toan electrostatic precipitator device at a temperature in the range of320°-420° C.
 17. The method of claim 15, wherein sulfuryl chloride isinjected into the duct prior to an electrostatic precipitator and/orfabric filter and/or scrubber at a temperature in the range of 100°-400°C.
 18. A mercury control system for a coal combustion system having acoal combustor and a pollutant control device, the mercury controlsystem comprising: a halogen dissociation unit for generating adissociated halogen gas, the halogen dissociation unit being incommunication with a flue gas stream of the coal combustion system; andwherein the mercury control system is configured to expose thedissociated halogen gas to mercury present in the flue gas stream,allowing the dissociated halogen gas to react with the mercury at atemperature in the range of 60°-400° C. and be captured by the pollutantcontrol device.
 19. The mercury control system of claim 18, wherein thepollutant control device is a particulate control device such as aelectrostatic precipitator and/or fabric filter and/or scrubber.
 20. Themercury control system of claim 18, wherein the halogen dissociationunit uses dissociated allyl iodide.
 21. The mercury control system ofclaim 18, wherein the halogen dissociation unit uses dissociated allylchloride.
 22. The mercury control system of claim 18, wherein thehalogen dissociation unit uses dissociated allyl bromide.
 23. Themercury control system of claim 18, further comprising a device forinjecting carbon particulates into the mercury control system.
 24. Themercury control system of claim 18, further comprising a photolysis lampfor exposing the dissociated halogen gas to light.
 25. The mercurycontrol system of claim 18, wherein the halogen dissociation unitoperates at a temperature in the range of 60°-400° C.